Yup. Your best bet would be to run the keys into the mixer, then come out of the mixer into a stereo crossover, with everything 100 hZ and below going to your K12 and everything above to the SS. Or, instead of the crossover and K12, dump another $299 into a Behringer B1200D or similar subwoofer that has stereo high-pass-filtered outputs (keys -> mixer -> sub -> SS).
Thanks, guys.
Yeah, I need to sell my QSC K12, pick up the B1200D.